Seasonal investigations of size-fractionated biomass and production were carried out from February 1992 to May 1993 in Jiaozhou Bay, China. Microplankton assemblages were separated into three fractions: pico-(0.7-2 μm), nano-(2-20 μ) and netplankton (20-200 μm). The biomass was measured as chlorophyll a (Chl a), particulate organic carbon (POC) and particulate organic nitrogen (PON). The production was determined by 1 4 C and 1 5 N tracer techniques. Averaged over the whole study period, the respective proportions of total biomass accounted for by net-, nano- and picoplankton were 26, 45 and 29% for Chl a, 32, 33 and 35% for POC, and 26, 32 and 42% for PON. The contributions to total primary production by net-nano- and picoplankton were 31, 35 and 34%, respectively.